In optical communication systems and optical interconnect systems, a technology that constructs an optical circuit by using an optical waveguide is important. And, for size reduction and electric power reduction of the systems, development of function integration in optical circuits is expected. In recent years, the attention drawn to a technology termed silicon photonics that employs a silicon optical waveguide as means for function integration in an optical circuit has been increasing.
In a silicon optical waveguide, by using silicon as a core and silica as a cladding and exploiting a high refractive index difference between the core and the cladding, and a fine core structure, strong light confinement effect can be obtained. By using such a silicon optical waveguide, an optical circuit that provides a high degree of integration is expected to be made. Furthermore, such a minute and highly integrated optical circuit can be fabricated on a large size wafer by exploiting process technologies accumulated for silicon LSI (large scale integration), which is also a major reason why silicon photonics is drawing attention.
FIG. 10 and FIG. 11 illustrate an example of a structure of a silicon optical waveguide. FIG. 10 is a diagram illustrating a general structure of a channel type optical waveguide. A channel structure 23 of silicon is formed in a silica layer 22 on a silicon substrate 21. The channel structure 23 functions as a core of the optical waveguide and the silica layer 22 functions as a cladding of the optical waveguide.
FIG. 11 is a diagram illustrating a general structure of a rib type optical waveguide. A silica layer 25, a silicon layer 26, and a silica layer 27 are stacked on a silicon substrate 24. A protruding structure 28 termed rib or ridge is formed on the silicon layer 26. In this structure, the light confinement in a direction perpendicular to the substrate is realized by a refractive index difference between silicon and silica. Furthermore, the light confinement in a direction parallel to the substrate is realized by an effective refractive index difference between thick silicon provided with the rib and thin silicon (slab) not provided with the rib.
In constructing highly integrated optical circuit by using a silicon optical waveguide, an element technology for realizing intersection of optical waveguides is very important. At an intersection point at which a plurality of optical waveguides intersect, it is difficult to avoid occurrence of diffraction in an optical signal propagating through an optical waveguide. PTL 1 and PTL 2 describe element structures for reducing the loss of an optical signal involved with diffraction.
FIG. 12 is a diagram illustrating a first example of a general optical circuit element that has an intersection point of optical waveguides. FIG. 12 is a diagram of a structure of cores of the optical waveguides that form the intersection point, viewed from an upper surface. One optical signal propagates from an incoming side 51 to an outgoing side 54 of an optical waveguide and another optical signal propagates from an incoming side 55 to an outgoing side 58. The optical waveguide that propagates the optical signal from the incoming side 51 to the outgoing side 54 and the optical waveguide that propagates the optical signal from the incoming side 55 to the outgoing side 58 intersect at an intersection point 59. The core width of each optical waveguide is enlarged at the intersection point 59. And, taper portions 52, 53, 56, and 57 are provided in the optical waveguides. The core width of the taper portions 52, 53, 56, and 57 is gradually enlarged toward the intersection point 59.
A fundamental mode 60 schematically illustrated in FIG. 12 illustrates a unimodal light intensity distribution in an optical waveguide core. In a portion of the optical waveguide whose core width is enlarged, the fundamental mode 60 propagating in the optical waveguide is less easily affected by changes in the width of a side wall of the core and the state of the side wall, so that the loss of the optical signal involved with diffraction at the intersection point 59 can be reduced.
In FIG. 12, with regard to the intersection of optical waveguides, illustration is made with a viewpoint focused on reduction of the loss of an optical signal that passes through a location of intersection. However, as the integration scale of an optical circuit increases, the influence of an optical signal passing through a plurality of intersection points provided on an optical waveguide becomes unignorable. For example, PTL 3 describes an optical circuit element in which a plurality of intersection points are disposed on a straight line on an optical waveguide.
FIG. 13 is a diagram illustrating a second example of a general optical circuit element that has intersection points of optical waveguides. FIG. 13 is a top view of a structure of optical waveguide cores that form the intersection points. A first optical signal propagates from an incoming side 61 to an outgoing side 62 of an optical waveguide. Furthermore, three optical waveguides intersect the optical waveguide in which the first optical signal propagates, at intersection points 69, 70, and 71, respectively. The interval between the intersection point 69 and the intersection point 70 is d1 and the interval between the intersection point 70 and the intersection point 71 is d2.
There are cases where the diffraction of an optical signal at an intersection of optical waveguides generates a fundamental mode that propagates in a direction opposing the optical signal, resulting in generation of reflected light. If, in such a case, a plurality of intersection points are disposed at equal intervals, an effect as a diffraction grating occurs in the optical circuit element and a loss dependent on wavelength occurs. To avoid this, the optical circuit element illustrated in FIG. 13 is provided with the interval d1 and the interval d2 that are different. A difference equal to or more than a certain value is provided between d1 and d2. Due to this, the optical circuit element illustrated in FIG. 13, while being incapable of reducing the loss at an intersection point at one location, can reduce the wavelength dependency of the loss that occurs due to a plurality of intersection points.